Fuel cells can be classified according to the type of electrolyte used: phosphoric acid fuel cell, alkaline fuel cell, molten carbonate fuel cell, solid oxide fuel cell, solid polymer electrolyte fuel cell, etc. Among them, the solid polymer electrolyte fuel cell, which is capable of low temperature operation and has high output density, has been commercialized as the power sources for automobiles and the home cogeneration systems.
Meanwhile, with the increasing sophistication of portable devices including laptop computers, cell phones and personal digital assistants (PDAs), power consumption has been increasing significantly in recent years. Since the energy density of lithium ion secondary batteries and nickel-metal hydride secondary batteries has failed to keep up with the increasing demand for the power consumption, there is a growing concern that the capacity of the power sources will be insufficient sometime soon.
Under the circumstances, attention has been given to the solid polymer electrolyte fuel cell (hereinafter referred to as “PEFC”) as the power source to solve the problem. Particularly, the direct oxidation fuel cell is considered most promising because the direct oxidation fuel cell can generate electric energy at room temperature by performing, on the electrode, the direct oxidation of a fuel without reforming the fuel to hydrogen and thus it does not require a reformer, and it can be made smaller.
As the fuel for the direct oxidation fuel cell, low molecular weight alcohols or ethers are investigated. Among them, methanol is considered most promising because the fuel cell utilizing methanol offers high energy efficiency and high output. The fuel cell utilizing methanol as the fuel is called “direct methanol fuel cell” (hereinafter referred to as “DMFC”).
The reactions on the anode and the cathode of the DMFC can be expressed by the following reaction formulas (1) and (2). The oxygen to be fed to the cathode as the oxidant is usually obtained from the air.CH3OH+H2O→CO2+6H++6e−  (1)3/2O2+6H++6e−→3H2O  (2)
Similar to the PEFC that utilizes hydrogen as the fuel, the DMFC currently utilizes, as the electrolyte membrane, a perfluorocarbon sulfonic acid membrane as typified by Nafion (registered trade name).
Ideally, methanol as the fuel reacts on the anode according to the formula (1), but methanol might pass through the electrolyte membrane to the cathode, which is called “crossover phenomenon”.
The crossover phenomenon reduces the power generation performance of the fuel cell. The methanol reaching the cathode is oxidized on the cathode according to the following formula (3). As a result, the potential of the cathode decreases to reduce the power generation voltage of the fuel cell.CH3OH+3/2O2→CO2+2H2O  (3)
The crossover phenomenon is believed to result from the combined use of a water-soluble fuel and an electrolyte membrane exhibiting proton conductivity when water is incorporated in the membrane, and it is a common problem for the direct oxidation fuel cells under development.
As for the theoretical electromotive force, the PEFC that utilizes hydrogen as the fuel has a theoretical electromotive force of 1.23 V, and the DMFC has a theoretical electromotive force of 1.21 V. In practice, however, because the reaction overvoltage of the formula (2) is so significant, the PEFC generates a voltage of 0.6 to 0.8 V. The DMFC generates a voltage of only 0.3 to 0.5 V because, in addition to the crossover phenomenon, the reaction overvoltage of the formula (1) is also significant.
For this reason, when fuel cells are used to operate electronic devices, it is necessary to connect a plurality of unit cells in series to form a stack, or to incorporate a charge pump circuit, to yield the desired voltage. Usually, the both methods mentioned above are combined for use.
In the case where a plurality of unit cells are connected, although it depends on the shape of the device utilizing the fuel cell as the power source and also on the presence or absence of auxiliary equipment such as a pump for transporting a fuel and a pump for transporting air, the arrangement of unit cells is determined mainly from a space-saving point of view. Usually, a stack comprising a plurality of flat unit cells is used. To be more specific, a single unit cell comprises an anode, a cathode, and an electrolyte membrane sandwiched between the anode and the cathode. A plurality of unit cells are stacked with bipolar plates or separators interposed therebetween such that a fuel channel is adjacent to the anode and an oxidant channel is adjacent to the cathode. The channels for fuel or oxidant are formed on the surfaces of the bipolar plates or separators.
Ideally, the fuel and air are fed uniformly into all the unit cells in the fuel cell stack described above. In practice, however, there is a variation in the supply of the fuel and air, causing each unit cell to exhibit different power generation performance, eventually reducing the power generation performance of the whole stack. In view of this, there is proposed to improve the structure of a manifold for supplying a fuel gas so as to feed the fuel gas uniformly to the unit cells (see Japanese Laid-Open Patent Publication No. Hei 05-190186).
Various techniques to prevent the variation of the fuel gas supply other than the above have been investigated from various aspects. Regarding the improvement of the flooding phenomenon at the cathode, however, improvement has been made only in the internal structure of a unit cell, and there is no focus on the improvement of the whole stack.
In the PEFC which is operated at a temperature of not higher than 100° C., i.e. the boiling point of water, the so-called flooding phenomenon is likely to occur. The flooding phenomenon is a phenomenon that impairs the gas diffusibility in the electrode because the water generated at the cathode due to the reaction of the formula (2) is supersaturated to aggregate. In order to overcome this problem, Japanese Laid-Open Patent Publication No. Hei 08-138696 proposes to change the shape of an oxidant gas channel within a unit cell to prevent the aggregation of water.
The incidence of the cathode flooding phenomenon varies depending on the unit cells. For example, when temperature differences occur among the unit cells in the stack, water is more likely to aggregate in the unit cells with a low temperature, increasing the incidence of the flooding phenomenon therein. In the case of a stack comprising a plurality of flat unit cells, the unit cell positioned at the top and that positioned at the bottom are likely to dissipate heat outside the stack. Accordingly, those cells have a relatively low temperature.
In the DMFC, because the incidence of the crossover phenomenon increases as the concentration of methanol is increased at the interface between the anode and the electrolyte membrane, an aqueous solution of methanol with a low concentration of 1 to 2 mol/L is often used. In the crossover phenomenon, however, water also passes through the electrolyte membrane with methanol. As such, a large amount of water is transported from the anode to the cathode. In some cases, the amount of water transported is about 100 times that of water generated at the cathode by power generation. Accordingly, the flooding phenomenon is an extremely serious problem in the DMFC.